Steam methane reforming is a method widely used for the production of hydrogen and/or carbon monoxide.
The steam reforming process is a well known chemical process for hydrocarbon reforming. In this process, a mixture of light hydrocarbons and steam (referred to as mixed feed or feed) reacts in the presence of a catalyst to form hydrogen, carbon monoxide and carbon dioxide. Several reactions are observed in the process, the most important chemical reaction observed in the steam reforming of light hydrocarbons—methane or natural gas (NG)—being the reaction of reforming of methane:CH4+H2O<—>CO+3H2 
The steam methane reforming (also known as SMR) gives a mixture with hydrogen (H2) and carbon monoxide (CO)— in presence of water vapor—as major components, but also CO2 as a minor component, CH4 as residual component and other components as traces.
The above reaction of reforming of methane with steam is an endothermic and slow reaction; it requires heat input, as well as catalyst to occur. The amount of reforming achieved depends on the temperature of the gas leaving the catalyst; exit temperatures in the range of 700°-950° C., possibly 1000° C. are typical for conventional hydrocarbon reforming.
A SMR unit is usually made of several vertical tubular reactors (or tubes) forming rows, placed in a furnace, filled with catalyst (usually in the form of pellets) and fed with a mixture of methane and steam.
Several typical furnace designs are encountered industry wide, mainly bottom, side and top fired; the two most often encountered designs are described hereafter:                the top-fired reformer design, with the burners located on the top of the furnace. It is one of the most referenced designs and is proposed by several technology providers;        the side fired reformer design, with the burners located in the side of the tube, on radiant wall.        
The main objective of the furnace design is to allow a proper heat transfer from the burner flames to the reforming tubes, while keeping below the tube design temperature. This temperature is a function of the tube mechanical load (mainly feed gas pressure), the mechanical properties of the alloys, and the desired lifetime of the tube.
Tubes are indeed a critical element with regards with the reliability of SMR furnace operation as their operating temperature results from a compromise between two antagonist objectives: a better process performance with increased temperature and good operation reliability with limited temperature (kept below design temperature).
Therefore, the tube operating temperature is a limiting parameter for furnace operations: the reformer has to be operated while keeping even the hottest tube below the design temperature.
FIGS. 1 to 3 illustrate typical SMR furnaces.
FIG. 1 illustrates a typical top-fired SMR furnace 101, with a refractory lined firebox 102 containing several rows of vertical tubes 103 with roof burners 104 being placed in rows in-between the tube rows. Tubes are filled with catalyst. A feed 105 (process fluid mixture (CH4+H2O)) entering through inlet headers is injected into the tubes where it reacts; the process gas flows through the catalyst in the tube from top to bottom and exits—as syngas—through outlet headers. The combustion products 106 out of the burners are usually blown vertically downwards, so that tube rows face the flames in their upper part, and a flue gases collector—not shown—is usually built at the furnace floor level which gathers the combustion products.
When the firebox contains many tubes (up to several hundreds) placed in rows, and due to necessary space for construction constraints—including structural beams—each of the row of tubes is split into several sections—it may occur that the sections contain different number of tubes facing the same number of burners, while receiving same power input; therefore, the total heating power received by one tube (or tube duty) is not equal for all tubes, being usually lower in the central section than in the extremity ones.
Moreover, the tubes located at the end of a tube section receive more heat than the other tubes of the same section, the reason being that end tubes are heated from a larger angular sector compared to those neighbored by two tubes. This transfer particularity can be also observed on side-fired furnaces, as described hereafter.
FIG. 2 illustrates a typical side fired SMR furnace 201, made of a refractory lined firebox 202 containing one single row of tubes 203. The burners 204 are horizontally aligned at different levels on the furnace walls, being therefore horizontally and vertically aligned; the combustion products (flue gases) 206 flowing out of the burners are blown vertically upward. A flue gases exhaust collector (not shown) is built at the furnace roof level.
FIG. 3a simulates the temperatures of the combustion products in a side fired SMR 201, showing that the symmetry of the burner 204 distribution induces that the combustion products (flue gases) converge to the center 307 of each square burner, thus creating a hot point. At this convergence point, the flow is forced into a re-circulation perpendicularly to the refractory towards the tubes 203; because just downstream the flames, the re-circulated gases are extremely hot, and induce hotspots 308 formation on the tubes impacted which constitutes a main drawback.
Taking into consideration the following points: (1) the process gas within the tubes flows from the top to the bottom of the firebox, counter currently to the flue gas flow, (2) the furnace is designed in order to distribute homogeneously the process gas to all the tubes, and (3) the process gas temperature increases during its flowing down along the tube length; this results, for each tube, in a tube temperature globally higher in the lower part of the furnace. Thus, the risk for tube damaging if the design temperature is exceeded is higher in this part of the tube.
This diagnosis of the problem results from simulation and is also confirmed by experimental tube temperature measurement profiles.
FIG. 3b illustrates the tubes temperatures and shows hot spots present in side fired furnace; a clear periodic pattern is identified, with hottest tubes being in the middle part between two burner rows and the cold tubes being in front of the burners.
In the case of the side-fired furnaces—as described above—the recirculation effect due to flue gas convergence implies that the tubes between the burner columns are more heated than the tubes situated right in front of the burners.
For the top fired furnaces as well, construction constraints imply that the tube duty is not identical for all the tubes in the combustion chamber.
There have been a number of attempts to improve the uniformity of the heating of the tubes in reformers.
It is known from FR 2850392 a process for the heat treatment of a hydrocarbon feedstock in which the feedstock to be treated circulates inside an exchange tube bundle that receives the heat emitted by radiant burners; the burners being placed in rows, —horizontally and vertically—the vertical firing profile is adapted so as to obtain determined heating profiles. A main drawback of this solution is a high investment cost with strong modification of the existing devices.
It is known from EP 1216955B the use of a variable heat flux side-fired burner system for use in processes for heating, reforming, or cracking hydrocarbon fluids or other fluids. In order to be flexible, the burner may be divided into multiple sections, flow rates being distributed for example along perforated plates with predetermined firing patterns. A main drawback of this solution is a high investment cost with strong modification of the existing devices; additionally, the solution does not avoid recirculation phenomena.
It is known from FR 2911600 a process for reforming hydrocarbons in a side-fired furnace, where the power of each burner is adjusted, burners of high power being placed near from burners of low power so as to reduce the accumulation of hot points on tubes.
It is known from US 2008286706 a heater and method of operation suitable for the cracking of hydrocarbons with under-stoichiometric firing in upper wall burners and over stoichiometric firing in the floor (hearth) wall burners to achieve the smoothest (flat) profile along the overall process length.
However, the solutions proposed by the prior art to achieve a more uniform heating of the tubes only attempt to mitigate the heat flux discrepancies from the combustion chamber side—either burner or flue gases or both of them.
Furthermore, prior art documents consider mainly the in-homogeneities of the temperature of a tube from top to bottom, but fail to take into account the tube temperature in-homogeneities between tubes. As a consequence, the solutions of the prior art fail to solve the problems of overheating of some tubes of the reformer compared to other tubes in the same reformer.
There is therefore a need for a solution that identifies the hottest and the coldest tubes in a furnace, and homogenizes the temperatures of all the tubes;                there is a need for a solution of the above problem that can be implemented on reformers in new installations, and on existing reformers during programmed shut down;        there is a need for a solution of the above problem that is a cheap solution and that does not impact negatively the production.        